Mechanism Of Separation Of Single-Walled Carbon Nanotubes By Conjugated Polymer

As one of the most studied carbon materials,single-walled carbon nanotubes have outstanding mechanical,thermal,optical,and electrical properties,and have been widely used in many fields such as composite materials,field effect transistors,sensors,organic photovoltaic,flexible electronic devices,and so on.Unfortunately,all known commercial preparation methods currently produce mixtures of SWCNT with different chirality and diameters.The noncovalent encapsulation method using conjugated polymers represented by polyfluorene has the advantages of simple operation and high selectivity,and is considered to be the most promising method for selective separation of SWCNTs.Undoubtedly,the interaction between conjugated polymers and SWCNT leads to selectivity for specific single-walled carbon nanotubes.Although a large number of experiments and theoretical simulations have attempted to explain the sorting mechanism of specific SWCNT,their conclusions are mostly a summary of phenomena and do not have broad universality.We still do not know the essential reason of chiral selectivity,and we also know little about the structural parameters that dominate the selectivity.It is still necessary to explore the interaction modes and selection mechanisms between polymers and SWCNT to design novel polymers for selective dispersion of specific SWCNT.In this paper,the semi empirical quantum chemistry method was used to systematically study the interaction mode between poly(9,9-dioctylfluorene)(PFO)and its derivatives poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-phenylene)](PFO-P)and(n,m)-SWCNT.The effect of structural factors such as the main chain and side chain of the polymer on the interaction mode of the system was emphatically explored.It was found that the main chain of the polymer tended to be distributed along the chiral direction of SWCNT to maximize dispersion interaction,The side chain arrangement of polymers affects the number of SWCNT adsorbed polymers and the number of stable configurations of the system.Under the guidance of thermodynamic process,they jointly determine the high selectivity of PFO and its derivatives to specific SWCNT.This paper mainly consists of the following parts:In chapter 1,the research background of this topic,as well as the research results and shortcomings achieved through experiments and theoretical simulations are introduced,leading to the significance and content of this topic.In chapter 2,the commonly used theoretical simulation methods are introduced,focusing on the semi empirical quantum mechanical method GFN2-xTB used in this topic,as well as weak interaction analysis methods such as molecular electrostatic potential,van der Waals potentials,energy decomposition analysis based on forcefield,and electron density difference.In chapter 3,a semi empirical quantum mechanical method GFN2-xTB was used to theoretically simulate the interaction system between PFO and its derivatives with(n,m)SWCNT.The interaction modes of single and multiple PFO adsorbed on the outer side of SWCNT were studied,and the role of PFO’s main and side chains in the selective mechanism of SWCNT was explored.We found that the interaction mode between PFO and SWCNT is essentially controlled by dispersion attraction,and the vdW potential can accurately describe the location and intensity of dispersion.Due to the chiral direction of SWCNT determining the arrangement direction of vdW potential,in order to maximize the dispersion interaction between PFO and SWCNT,the main chain of PFO tends to distribute along the chiral direction of SWCNT.For SWCNT with large chiral angles,this interaction mode has higher thermodynamic stability,which is conducive to the selective dispersion of PFO towards SWCNT with large chiral angles.In addition,when multiple PFO are wrapped on the outer side of SWCNT,the octyl side chains between adjacent PFO can form a cross finger shaped regular structure,which determines the maximum amount of PFO adsorbed on the outer side of a specific(n,m)-SWCNT and the number of stable structures in the(n,m)-SWCNT/iPFO composite system.Combined with the thermodynamic data of the composite system,we found that the diameter selectivity of PFO towards SWCNT is essentially driven by the number of stable configurations of each composite system and their thermodynamic stability.In chapter 4,a semi empirical quantum mechanical method was used to study the weak interaction system composed of PFO-P and SWCNT,and the possible reasons for the differences in the selectivity of PFO-P and PFO to SWCNT were discussed.We found that the introduction of phenyl can significantly increase the flexibility of the PFO-P backbone,which is crucial for maintaining the thermodynamic stability of the SWCNT/PFO-P composite system.Unlike PFO,when PFO-P is adsorbed on the outside of SWCNT with a small chiral angle,the main chain of PFO-P can still be distributed along the chiral direction of single-walled carbon nanotubes to maximize dispersion and attraction,and the complex with this configuration exhibits the highest thermodynamic stability,which may be the reason why the chiral angle range of products obtained by PFO-P sorting SWCNT becomes larger.